Besides the ball valves with full openings, there are ball valves with a reduced opening on the market, wherein the flow openings of the valve ball are, in other words, smaller than the diameter of the flow channel formed by the tubes. In some of the valve structures of this type there is not even any separate casing around the ends of the tubes and the valve ball, the casing being integrated with the tube parts formed by the flow channel provided by the valve. These well known solutions have been described, e.g. in the FI declaration publication 71609 and in the U.S. Pat. No. 4,545,564.
According to the state of art technology, the manufacturing of a ball valve with full opening takes place so that, in order to provide the sealing box required by the seals between the tubes and the valve ball, in the ends of the tubes, the inner diameter of which corresponds to the diameters of the flow openings, interior widenings are cut by lathe. The casing around the valve ball and the tube ends is fixed by welding to the sides of the tubes so that the sealing boxes will stay inside the space defined by the casing. An example of this known solution is presented, among others, in the FI patent application 3532/72. The solution requires that the casing of the tube has to be thick enough to enable the forming of a sealing box within it. In practice, the process has been followed so that the casing in the tube end of the valve has been thicker than in other parts of the tube, however, resulting in the disadvantage of heavy structure and in high costs of manufacturing. Furthermore, in connection with the welding of the casing on the valve ball and the tubes, there has been a danger of welding splashes entering under the casing where they can harm the movement of the valve ball that is the basis of the operation of the valve.
The object of this invention is to provide a solution with which said problems of known ball valves with full opening are eliminated. The manufacturing method of the valve according to the invention is characterized by that, in both tube ends, a substantially stepped protruding part is formed surrounding the ends, including an annular sealing, that the tube ends and the sealings are brought against the valve ball on its opposite sides, that the sleeve-like ends of the casing are reduced by the pressure applied to them to make their inner diameters match with the outer diameters of the tubes, and that, finally, the ends of the casings are attached to the sides of the tubes inserted in the casings.
The solution according to the invention makes the valve structure lighter and saves material compared with the known ball valves with full opening. Also, the need of forming and machining the tube ends is reduced. When the fixing points of the valve casing are located, as seen from the valve ball, behind the stepped extensions of the heads of the tubes the welding seams around the tubes attaching the casing are shortened, while the stepped protrusions of the tube ends operate as splash guards that makes the entry of welding splashes inside the space defined by the casing more difficult.